A submarine cable system having the OADM (Optical Add/Drop Multiplex) function of freely inserting or removing signals for each light wavelength includes a plurality of paths, which are accommodated in one optical fiber to improve the flexibility of a communication network and thereby relieve the burden of plant and equipment investment.
In the submarine cable system having the OADM function, the total power of a signal transmitted through cables made of optical fibers is set constant. More specifically, in the submarine cable system having the OADM function, when some wavelength components of a signal are lost upon, for example, cable disconnection, the remaining wavelength components of the signal are amplified to maintain the total power of the signal constant.
However, when the powers of specific wavelength components of the signal reach a predetermined value or more upon amplification of only these powers, the optical spectrum changes due to factors such as deterioration of the waveform of the signal related to the nonlinear effects of the optical fibers, thus degrading the transport quality of the signal.
Thus, the submarine cable system has the OADM Fault Recovery function of, when trouble occurs in any cable, compensating the intensity (power) level of a signal group remaining without a loss, using dummy light to ensure a given communication quality.
PTL 1 discloses a technique for compensating the intensity (power) level of a signal (group) in an optical communication system. To compensate for the difference in power level between a signal (group) from a transmitting station and a signal (group) from a branch station, the optical communication system multiplexes the signal (group) transmitted from the branch station with a dummy signal to compensate the signal (group) from the branch station using dummy light. The compensation allows the signal (group) from the transmitting station and the signal (group) from the branch station to keep nearly the same power level to prevent degradation of the system operating characteristics.
However, in the optical communication system described in PTL 1, since the entire signal (group) from the branch station is multiplexed with dummy light, the power level of the entire signal (group) can be compensated but the power levels of, for example, some components of the signal (group) cannot be compensated.
In contrast, to overcome this situation, PTL 2 discloses a technique for providing dummy light generation/adjusting units in terminal apparatuses (transmitting stations) to compensate the power level of the signal for each terminal apparatus (transmitting station). PTL 2 describes providing each terminal apparatus (transmitting station) with a dummy light generation/adjusting unit which generates dummy light corresponding to a portion suffering optical signal disconnection upon the occurrence of cable disconnection trouble to maintain the channel power of the signal constant.
In the submarine cable system, since a plurality of paths for connecting different locations to each other can be accommodated in only one optical fiber, signals from a plurality of terminal apparatuses (transmitting stations) can be multiplexed and transmitted to an opposed terminal apparatus (receiving station). Therefore, the power levels of some components of a signal (group) obtained by multiplexing signals from a plurality of terminal apparatuses can be compensated by compensating the power level of the signal using dummy light for each terminal apparatus, as in PTL 2.